Method for fabricating ultra short channel PMOSFET with buried source/drain junctions and self-aligned silicide

ABSTRACT

The proposed method of the present invention forms MOSFETs with improved short channel effects and operating speeds over conventional devices. The method for fabricating MOSFETs includes the following steps. At first, isolation regions are formed on a semiconductor substrate and a gate insulating layer is formed on the substrate. A first conductive layer is then formed on the gate insulating layer and a first dielectric layer is formed on the first conductive layer. A removing process is performed to remove portions of the gate insulating layer, the first conductive layer and the first dielectric layer to define a gate structure. A layer formation step is carried out to form a thermal oxide layer on the substrate and on sidewalls the first conductive layer. Doped dielectric sidewall spacers are then formed on sidewalls of the gate structure. A removing step is carried out to remove portions of the thermal oxide layer uncovered by the doped dieletric sidewall spacers. Next, the first dielectric layer is removed and a doped silicon layer is selectively deposited on the first conductive layer and on exposed regions of substrate surface. A first metal layer is then formed on the substrate. A thermal process is carried out to drive in dopants in the doped dielectric sidewall spacers and the doped silicon layer, and to convert portions of the first metal layer into a metal silicide layer lying over the doped silicon layer. A removing step then removes unreacted portions of the first metal layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This invention is a continuation-in-part application of an application filed with a Ser. No. of 09/025,969 filed Feb. 19, 1998, now U.S. Pat. No. 6,063,680 issued May 16, 2000 under the title of “MOSFETs with a Recessed Self-Aligned Silicide Contact and an Extended Source/Drain Junction”, and assigned to the same assignee with the same inventor as the present application.

FIELD OF THE INVENTION

The present invention relates the fabrication process of semiconductor devices, and more specifically, to a method for fabricating a P-type metal oxide semiconductor field effect transistor (PMOSFET) with buried source/drain junctions and self-aligned silicide.

BACKGROUND OF THE INVENTION

From the birth of the first integrated circuit in 1960, the number of devices on a chip has grown at an explosively increasing rate. The progress of the semiconductor integrated circuits has stepped into the ULSI (ultra large scale integration) level or even higher level after almost four decades of development. The capacity of a single semiconductor chip has increased from several thousand devices to hundreds of million devices, or even billions of devices. Integrated circuit devices like transistors, capacitors, and connections must be greatly narrowed accompanying with this advancement. The increasing packing density of integrated circuits generates numerous challenges to the semiconductor manufacturing process. Every element or device needs to be formed within a smaller area without influencing the characteristics and the operations of the integrated circuits. The demands on high packing density, low heat generation, and low power consumption devices with a good reliability and a long operation life must be maintained without any degradation in function. These achievements are expected to be reached with the simultaneous developments and advancements in the photography, the etching, the deposition, the ion implantation, and the thermal processing technologies, namely the big five key aspects of semiconductor manufacturing. The continuous increase in the packing density of the integration circuits must be accompanied by a shrinking minimum feature size. With present semiconductor manufacturing technology, the processes with a generally one-third micrometer in size is widely utilized. For making the next generation devices, the technologies focusing mainly on one-tenth micrometer or even nanometer sizes are highly demanded.

Transistors, or more particularly metal oxide semiconductor field effect transistors (MOSFET), are the most important and frequently employed devices in the integrated circuits with the high performance. However with the continuous narrowing of device size, the sub-micron scale MOS transistors have to face many risky challenges. As the MOS transistors become narrower and thinner accompanied by shorter channels, problems like the junction punchthrough, the leakage, and the contact resistance, cause the reduction in the yield and reliability of the semiconductor manufacturing processes.

The self-aligned silicidation technology is a vital application to improve the operation speed of the ULSIIVLSI MOS devices in manufacturing the sub-micron feature size semiconductor devices. Unfortunately, there exists some trade-off in employing the technologies like self-aligned silicide. In general, the self-aligned silicidation process results in a high junction leakage coming from the metal penetration. The metal penetration into the silicon substrate spikes the junction and/or the residual metal to cause the leakage problem. The silicide across the LDD spacer, which is not totally removed after the salicidation, causes the bridge between the adjacent devices like the gate and the source/drain regions. The detailed negative effects of the self-aligned silicidation technology on sub-micrometer devices are illustrated in the article by C.Y. Lu et al. (“Process Limitation and Device Design Tradeoffs of Self-Aligned TiSi2 Junction Formation in Submicrometer CMOS Devices”, in IEEE Trans. Electron Devices, vol. ED-38, No. 2, 1991) The device design tradeoffs for a shallow junction with a salicide structure are proposed. Process limitations of both junction formation schemes for sub-micrometer application and future scaling down are also established in the work.

In the present fabrication process, the self-aligned silicide (SALICIDE) technology is widely use to increase the packing density of ULSI circuits and to reduce the interconnect resistance for high speed operation. One of the articles relates to the self-aligned silicide (SALICIDE) technology is the work of K. Fujii et al, titled “A Thermally Stable Ti—W Salicide for Deep-Submicron Logic with Embedded DRAM” (IEEE, IEDM 96-451, 1996). The above article states that Ti-5%W salicide has high-thermal stability up to 800° C. as well as sheet resistance for 0.18 μm devices.

As for “short channel effect”, it could be improved by using the extended ultra-shallow source/drain junction. One of the articles relating to the problems is proposed by A. Hori et al. in their work titled “A 0.05 μm-CMOS with Ultra Shallow Source/Drain Junctions Fabricated by 5 Kev Ion Implantation and Rapid Thermal Annealing” (IEEE, IEDM 94-485, 1994). A deep submicron PMOSFET has been fabricated together with a NMOSFET. In the proposed process, ultra shallow source/drain junctions were developed on the basis of 5 KeV ion implantation technology and rapid thermal annealing. The short channel effect was successfully suppressed and the delay time per stage of unloaded CMOS inverter is improved at the supply voltage of 1.5 V.

SUMMARY OF THE INVENTION

The proposed method of the present invention forms a P-type metal oxide semiconductor field effect transistor (PMOSFET) with buried source/drain junctions and self-aligned silicide. The application of self-aligned metal silicide source drain contact, in combination with the metal silicide gate contact, raises the operation speed of the transistors. The structure of the extended ultra-shallow source/drain junctions improves the short channel effects in the conventional devices. The packing density of transistors in the integrated circuit can be raised significantly with improved structure formed with the method of the present invention.

The method for fabricating metal oxide semiconductor field effect transistors (MOSFETs) includes the following steps. At first, isolation regions are formed on a semiconductor substrate and a gate insulating layer is formed on the substrate. A first conductive layer is then formed on the gate insulating layer and a first dielectric layer is formed on the first conductive layer. A removing process is performed to remove portions of the gate insulating layer, the first conductive layer and the first dielectric layer to define a gate structure. A film formation step is carried out to form a thermal oxide layer on the substrate and on sidewalls of the first conductive layer. Doped dielectric sidewall spacers are then formed on sidewalls of the gate structure. A removing step is carried out to remove portions of the thermal oxide layer uncovered by the doped dielectric sidewall spacers.

Next, the first dielectric layer is removed and a doped silicon layer is selectively deposited on the first conductive layer and on exposed regions of substrate surface. A first metal layer is then formed on the substrate. A thermal process is carried out to to drive in dopants in the doped dielectric sidewall spacers and the doped silicon layer, and to convert portions of the first metal layer into a metal silicide layer lying over the doped silicon layer. The dopants in the doped dielectric sidewall spacers are driven into the substrate to form the extended source/drain junctions, and the dopants in the doped silicon layer are driven into the substrate to form source/drain junctions. A removing step then removes unreacted portions of the first metal layer.

In another preferred embodiment of the present invention, a removing step can be performed, after removing portions of the thermal oxide layer uncovered by the doped dielectric sidewall spacer to remove portions of a substrate surface, in order to form recessed regions on the substrate in regions uncovered by the gate structure and the doped dielectric sidewall spacers, preferably by directly etching a substrate material.

In addition to the aforementioned process of forming MOS transistors, several subsequent steps are typically performed to form interconnections. Firstly, a second dielectric layer is formed on the substrate and an annealing process is performed to the substrate. Portions of the second dielectric layer are removed to form contact holes. A second metal layer is then formed within the contact holes and on the second dielectric layer. Finally, portions of the second metal layer are removed to define interconnections with the remaining metal paths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of forming a silicon oxide layer, a nitride layer and a field oxide region on a silicon substrate in accordance with the present invention.

FIG. 2 is a cross-sectional view of defining a gate structure of transistors in accordance with the present invention.

FIG. 3 is a cross-sectional view of forming a thermal oxide layer on the silicon substrate and on the sidewalls of the first conductive layer in accordance with the present invention.

FIG. 4 is a cross-sectional view of forming doped dielectric spacers on sidewalls of the gate structure in accordance with the present invention.

FIG. 5 is a cross-sectional view of removing the first dielectric layer and forming recessed regions through direct etching the substrate in accordance with the present invention.

FIG. 6 is a cross-sectional view of forming a selectively-deposited, doped silicon layer and a first metal layer on the substrate in accordance with the present invention.

FIG. 7 is a cross-sectional view of forming a metal silicide layer on the doped silicon layer in accordance with the present invention.

FIG. 8 is a cross-sectional view of depositing a second dielectric layer and defining interconnections in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses a method of fabricating a P-type metal oxide semiconductor field effect transistor (PMOSFET) with buried source/drain junctions and self-aligned silicide. Recessed source/drain regions are optionally formed with an etching process with reduced etching damage and improved selectivity to other dielectrics such as oxide layers. A self-aligned silicide contact is formed without damaging bridging effect in the conventional processes, thus the operation speed of the devices can be significantly improved. With the extended source/drain junctions formed by the method of the present invention, the short channel effects, which are major problems encountered by the devices of reduced size in the ULSI stage, can be suppressed by the ultra shallow extended junctions in the present invention.

The method and the steps in the present invention applied on a semiconductor wafer can create different types of transistors like the NMOS and the PMOS transistors, and also with a greater number of devices at a time. For a clear illustration, the steps for forming a single PMOS transistor are illustrated in detail. The NMOS transistors can also be fabricated by applying the similar method. Since the variations in the processes for incorporating the formation of the NMOS transistors are well known in the art, the details are not described. Referring to FIG. 1, a semiconductor substrate 2, preferably composed of single crystalline silicon in a <100> direction, is provided. A silicon oxide layer 4 is grown on the silicon substrate 2 by a thermal oxidation process. A silicon nitride layer 6 is deposited on the silicon oxide layer 4, typically by a chemical vapor deposition process like low pressure chemical vapor deposition (LPCVD). Next, portions of the silicon nitride layer 6 and of the silicon oxide layer 4 are removed via lithography and etching steps to define regions for forming isolation regions. Isolation regions such as field oxide (FOX) regions 8 are then grown on the substrate 2 through the employment of thermal oxidation process. The field oxide regions 8 provide isolation between devices on the substrate 2.

Referring to FIG. 2, the silicon oxide layer 4 and the silicon nitride layer 6 are removed via wet etching step. Then, a gate insulating layer 10, preferably a silicon oxide layer in the case, is grown on the substrate 2, typically by a thermal oxidation process. Then, a first conductive layer 12 is formed on substrate 2, including the regions over the field oxide region 8 and the gate insulating layer 10. Next, a first dielectric layer 14 is formed on the first conductive layer 12. Following the formation of the three stacked layers 10, 12 and 14, portions of the gate insulating layer 10, the first conductive layer 12 and the first dielectric layer 14 are removed via lithography and etching steps, in order to define a gate structure 16 as shown in the figure.

In the preferred embodiments, the first conductive layer 12 is a doped polysilicon layer. The thickness of the first conductive layer 12 is about 500 to 5000 angstroms. The first dielectric layer 14 is preferably a silicon nitride layer, which is also served as an anti-reflection layer in a later lithography process for defining gate regions. The silicon nitride layer can be formed with a chemical vapor deposition. Alternatively, a silicon oxide layer can also be employed as the first dielectric layer 14, which is typically formed with a major reacting gas of tetra-ethyl-ortho-silicate (TEOS) in a chemical vapor deposition process. The thickness of the first dielectric layer 14 is about 100 to 1000 angstroms.

Referring to FIG. 3, a thermal oxide layer, which includes two separate portions 18 and 20 as indicated in the figure, is grown on the substrate 2 and also on the sidewalls of the first conductive layer 12 by a thermal oxidation process. The thermal oxide layer 18 on the substrate 2 is preferably grown in an N₂O or NO ambient, in order to recover the etching damages of the substrate surface remained from the previous etching step. Preferably, the thickness of the thermal oxide layer 18 is about 20 to 200 angstroms and the thickness of the thermal oxide layer 20 is about 40 to 250 angstroms.

Turning to FIG. 4, doped dielectric sidewall spacers 22 are then formed on the sidewalls of the gate structure 16. Generally, the doped dielectric sidewall spacers 22 are formed by forming and etching back a doped oxide layer to have the doped spacers. For forming extended source/drain junctions of the PMOS transistor in the underlying substrate 2, borosilicate glass (BSG) spacers are preferably formed. In the preferred embodiments, a BSG layer is firstly deposited to cover over the field region 8, the thermal oxide layer 18 and the gate structure 16. Next, portions of the BSG layer is removed to form the doped dielectric spacers 22 by an etch back process, and portions of the thermal oxide layer 18, which are located over the substrate 2 and uncovered by the doped dielectric spacers 22, are preferably removed within the etching back process. In the preferred embodiments of the present invention, the thickness of the BSG layer is about 500 to 2500 angstroms.

Referring to FIG. 5, by adding an optional step in the method of the present invention, portions of a substrate surface uncovered by the gate structure 16 and the doped dielectric sidewall spacers 22 can be removed to form recessed regions on the substrate 2. In the preferred embodiments, the removal of portions of the substrate 2 are performed with an selective and isotropic etching to remove it portions of the substrate material, such as silicon, with raised selectivity to other exposed dielectric materials, such as silicon oxide and silicon nitride. The depth of the recessed regions is between about 0 to 1000 angstroms, wherein the substrate partially removing step is not performed to have depth of the recessed regions of 0 angstroms. the An example of the selective and mainly isotropic etching process is a down-flow etching technique performed preferably with fluorine radicals. In the work of Y. Mitani et al. titled “Buried Source and Drain (BSD) Structure for Ultra-shallow Junction Using Selective Deposition of Highly Doped Amorphous Silicon” (p. 176-177, 1996 Symposium on VLSI Technology Digest of Technical Papers, IEEE), the selective and isotropic etch is illustrated in detail, providing high selectivity to silicon dioxide and low damages to the silicon surface of the substrate 2.

Alternatively, the method of the present can also form the PMOS transistor without employing aforementioned step of forming the recessed regions. Following the partial substrate removal or the partial thermal oxide layer 18 removal step, the first dielectric layer 14 located on the top of the gate structure 16 is then removed. In the case of using silicon nitride as the first dielectric layer 14, the first dielectric layer 14 is typically removed by a hot H₃PO₄ solution in a wet etch process.

Turning FIG. 6, a doped silicon layer 24 is then formed selectively on the first conductive layer 12 and on exposed regions of substrate surface. The doped silicon layer is preferably either an expitaxial silicon layer or an amorphous silicon layer. A selective silicon deposition process with in-situ doped P-type dopants, such as boron-based dopants, is carried out to form a doped amorphous silicon layer on the first conductive layer 12 and on the exposed regions of the substrate surface. Alternatively, a selective silicon deposition process, employing an ultra highvacuum chemical vapor deposition (UHVCVD) with in-situ doped P-type dopant, such as boron-based dopants, can be to form a doped amorphous silicon layer or a doped expitaxial silicon layer on the first conductive 12 layer and on the exposed regions of the substrate surface. The doped ions in the selectively doped silicon layer 24 provide a diffusion source of dopants to form source/drain junctions and extended source/drain junctions in the underlying substrate region.

A first metal layer 26 is deposited on the silicon substrate 2. In the preferred embodiments of the present invention, the first metal layer 26 can be refractory metals or noble metals like Ti, W, Co, Pt, Ni, Cr, Pd, Zr and the like. The first metal layer 26 is deposited either by PVD sputtering or CVD systems. The thickness of the first metal layer 26 is about 50 to 1000 angstroms.

Referring to FIG. 7, a silicidation process is performed to convert portions of the first metal layer 26 and the contacting silicon materials, such as the doped silicon layer 24 in the case, into a metal silicide layer 28. Therefore, portions of the first metal 26, which are located on the isolation region 8 and on the doped dielectric spacers 22, are remained unreacted after the silicidation process under a raised temperature between about 350 to 700° C. Under the thermal processing of the silicidation process, the dopants in the doped dielectric sidewall spacers 22 and the doped silicon layer 24 are further drove into the underlying substrate 2 and activated. The dopants in the doped dielectric sidewall spacers 22 are drove into the substrate 2 to form the extended source/drain junctions 30, and the dopants in the doped silicon layer 24 are drove into the substrate 2 to form source/drain junctions 27, and are also downwardly diffused to further dope the first conductive layer 12 of polysilicon. In the preferred embodiments, ultra shallow source/drain junctions 27 are formed to have a dose between about 5E14 to 5E16 atoms/cm², and ultra shallow extended source/drain junctions 30 are formed to have a dose between about 5E13 to 2E15 atoms/cm².

The unreacted portions of the first metal 26 are then removed. Preferably, the unreacted portions of the first metal 26 are removed by well known chemical wet etch. Therefore, the MOS transistor with a recessed self-aligned silicide contact and extended source/drain junctions is formed.

In addition to the aforementioned processes, several additional steps can be added to finish one or more layers of interconnections between devices. Turning to FIG. 8, a second dielectric layer 32 is then deposited, typically with chemical vapor deposition techniques. A silicon oxide based layer, like a silicon dioxide layer, can be used as the second dielectric layer 32. A thermal treatment, such as an annealing process in the case, can be performed to condense the second dielectric layer 32, and to anneal the metal silicide layer 28 into a stable phase. A well known rapid thermal process (RTP) can be employed in the preferred embodiments.

Next, portions of the second dielectric layer 32 are removed via lithography and etching processes to form contact holes within the second dielectric layer 32, extending down to the gate and the source/drain regions. A second metal layer 34 is deposited on the second dielectric layer 32 and filled into the contact holes. Finally, portions of the second metal layer 34 are removed to define interconnections between various devices on the substrate 2. In the preferred embodiments, the second metal layer 34 can be conductive materials like Al, W, Cu, Ti, Pt, Cr, Ni, and their compounds or combinations.

The proposed method in the present invention forms a P-type metal oxide semiconductor field effect transistor (PMOSFET) with buried source/drain junctions and self-aligned silicide. The application of self-aligned silicide source drain contacts, in combination with the silicide gate contacts, raises the operation speed of the transistors. The structure of the extended ultra-shallow source/drain junctions improves the short channel effects in the conventional devices. The packing density of transistors in integrated circuit can be raised significantly with improved structure formed with the method of the present invention.

As is understood by a person skilled in the art, the foregoing descriptions of the preferred embodiment of the present invention is an illustration of the present invention rather than a limitation thereon. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. The scope of the claims should be accorded to the broadest interpretation so as to encompass all such modifications and similar structures. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 

I claim:
 1. A method for fabricating a P-type metal oxide semiconductor field effect transistor (PMOSFET) with buried source/drain junctions and self-aligned silicide, said method comprising the steps of: forming isolation regions on a semiconductor substrate; forming a gate insulating layer on said substrate; forming a first conductive layer on said gate insulating layer; forming a first dielectric layer of silicon nitride on said first conductive layer; removing portions of said gate insulating layer, said first conductive layer and said first dielectric layer to define a gate structure; forming a thermal oxide layer on said substrate and on sidewalls of said first conductive layer; forming doped dielectric sidewall spacers of borosilicate glass (BSG) on sidewalls of said gate structure; removing portions of said thermal oxide layer uncovered by said doped dielectric sidewall spacers; removing said first dielectric layer; forming a doped silicon layer selectively on said first conductive layer and on exposed regions of substrate surface, said silicon layer containing p-type dopants; forming a first metal layer on said silicon layer; performing a thermal process, wherein said thermal process simultaneously drives dopants in said doped dielectric sidewall spacers into said semiconductor substrate there under to form extended source/drain junctions and drives dopants in said doped silicon layer into said semiconductor substrate there under to form source/drain junctions, and converts said silicon layer into a metal silicide layer; removing unreacted portions of said first metal layer; forming a second dielectric layer on said substrate; performing an annealing process to said substrate; removing portions of said second dielectric layer to form contact holes; forming a second metal layer within said contact holes and on said second dielectric layer; and removing portions of said second metal layer to define interconnections.
 2. The method of claim 1, after removing said portions of said thermal oxide layer uncovered by said doped dielectric sidewall spacers, further comprises a step of removing portions of a substrate surface to form recessed regions on said substrate in regions uncovered by said gate structure and said doped dielectric sidewall spacers, by directly etching a substrate material.
 3. The method of claim 2, wherein said recessed regions are formed by a selective isotropic etch to remove portions of said substrate material with a selectivity to said doped dielectric sidewall spacers and said first dielectric layer.
 4. The method of claim 1, wherein said first conductive layer comprises doped polysilicon.
 5. The method of claim 1, wherein said doped silicon layer is formed with a selective silicon deposition process with in-situ doped P-type dopants to form a doped amorphous silicon layer on said first conductive layer and on said exposed regions of said substrate surface.
 6. The method of claim 1, wherein said doped silicon layer is formed with a selective silicon deposition process, employing a ultra high-vacuum chemical vapor deposition (UHVCVD) with in-situ doped P-type dopants to form a doped amorphous silicon layer or a doped expitaxial silicon layer on said first conductive layer and on said exposed regions of said substrate surface.
 7. A method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) with buried source/drain junctions and self-aligned silicide, said method comprising the steps of: forming isolation regions on a semiconductor substrate; forming a gate insulating layer on said substrate; forming a first conductive layer on said gate insulating layer; forming a first dielectric layer on said first conductive layer; removing portions of said gate insulating layer, said first conductive layer and said first dielectric layer to define a gate structure; forming a thermal oxide layer on said substrate and on sidewalls of said first conductive layer; forming doped dielectric sidewall spacers on sidewalls of said gate structure; removing portions of said thermal oxide layer uncovered by said doped dielectric sidewall spacers; removing said first dielectric layer; forming a doped silicon layer selectively on said first conductive layer and on exposed regions of substrate surface; forming a first metal layer on said substrate; performing a thermal process, wherein said thermal process simultaneously drives dopants in said doped dielectric sidewall spacers into said semiconductor substrate there under to form extended source/drain junctions and drives dopants in said doped silicon layer into said semiconductor substrate there under to form source/drain junctions, and converts said silicon layer into a metal silicide layer; removing unreacted portions of said first metal layer.
 8. The method of claim 7, after removing said portions of said thermal oxide layer uncovered by said doped dielectric sidewall spacers, further comprises a step of removing portions of a substrate surface to form recessed regions on said substrate in regions uncovered by said gate structure and said doped dielectric sidewall spacers, by directly etching a substrate material.
 9. The method of claim 8, wherein said recessed regions are formed by a selective isotropic etch to remove portions of said substrate material with a selectivity to said doped dielectric sidewall spacers and said first dielectric layer.
 10. The method of claim 7 further comprises the steps of: forming a second dielectric layer on said substrate after said unreacted portions of said first metal layer are removed; performing an annealing process to said substrate; removing portions of said second dielectric layer to form contact holes; forming a second metal layer within said contact holes and on said second dielectric layer; and removing portions of said second metal layer to define interconnections.
 11. The method of claim 10 further comprises a step of removing said doped dielectric sidewall spacers before forming said second dielectric layer.
 12. The method of claim 10, wherein said second dielectric layer comprises silicon oxide.
 13. The method of claim 7, wherein said gate insulating layer comprises silicon oxide.
 14. The method of claim 7, wherein said first conductive layer comprises doped polysilicon.
 15. The method of claim 7, wherein said first dielectric layer comprises silicon nitride.
 16. The method of claim 7, wherein said doped dielectric sidewall spacers comprises borosilicate glass (BSG) spacers.
 17. The method of claim 7, wherein said first metal layer is selected from the group consisting of Ti, W, Co, Pt, Ni, Cr, Pd and Zr.
 18. The method of claim 7, wherein said doped silicon layer is composed a material selected from the group consisting of expitaxial silicon and amorphous silicon.
 19. The method of claim 7, wherein said doped silicon layer is formed with a selective silicon deposition process with in-situ doped P-type dopants to form a doped amorphous silicon layer on said first conductive layer and on said exposed regions of said substrate surface.
 20. The method of claim 7, wherein said doped silicon layer is formed with a selective silicon deposition process, employing a ultra high-vacuum chemical vapor deposition (UHVCVD) with in-situ doped P-type dopants to form a doped amorphous silicon layer or a doped expitaxial silicon layer on said first conductive layer and on said exposed regions of said substrate surface. 